Flying spot scanners, often referred to as raster output scanners (ROS), conventionally have a reflective multi-faceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction (also known as the fast-scan direction) while the recording medium is being advanced in an orthogonal, or process, direction (also known as the slow-scan direction) such that the beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with a binary sample string, whereby the recording medium is exposed to the image represented by the samples as it is being scanned.
Laser arrays of multiple wavelength sources have many important applications. For example, a color xerographic printer that uses four different wavelength beams can have significantly higher throughput than a color xerographic printer that uses only one laser beam. This is so because a four wavelength laser printer can produce overlapping beams, sweep those beams using a single raster output polygon scanner and a single set of optics, subsequently separate the individual beams using wavelength selective filters, and direct each beam onto a separate xerographic imaging station. A latent image for each wavelength is then developed and a full color image is obtained by transferring the developed images onto a single recording medium. Such a system is derscribed in U.S. Pat. No. 5,243,359 to Fisli. In another application, multiple wavelength overlapping beams are imaged without separation at a single imaging station. Once again, the multiple beams allow higher throughput than a single beam. Such a system is described in U.S. Pat. No. 5,373,313 to Kovacs.
A diode laser package with closely spaced emitters would allow a single set of optics to be used, and would eliminate the need for beam combining optics. However, the individual laser diodes in such a package should be closely spaced (preferably spaced as close as 50 .mu.m) to avoid off-axis distortion effects as the beams propagate through the optical system.
While multiple wavelength laser sources are advantageous, the use of multiple wavelengths creates its own set of problems. For example, the focal length of a laser beam through a given set of optics is wavelength dependent. Thus, if a single set of optics is used in a multiple wavelength system, the different wavelength laser beams will have different focal lengths. In a printer, different focal lengths will result in multiple focal planes for the imaged spots if all laser beams emanate from the same plane. Different focal positions can cause various registration problems which are highly undesirable.
Both ROS and multi-beam printer techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura, the disclosure of which is incorporated herein by reference. In the Kitamura patent, multiple laser sources are arranged diagonally (see FIG. 10b of the Kitamura patent) to sweep multiple beams across a single photoreceptor. The beams are also displaced from each other in the cross-scan direction so that multiple lines can be scanned simultaneously across the photoreceptor. An object of the Kitamura patent is to reduce variations in pitch by spacing individual laser sources within the laser array closely in a compact structure.
Commonly assigned U.S. Pat. No. 5,243,359 to Fisli, the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS system suitable for deflecting multiple laser beams in a multi-station printer. In the Fisli patent, the rotating polygon mirror simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams having their largest divergence angles parallel to one another. The reflected laser beams are subsequently separated by a plurality of optical filters and directed to their associated imaging stations. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar path lengths for each beam. This is facilitated by locating all lasers in one integral unit. The laser diodes are arranged in a line in a cross-scan direction (sagitally offset), i.e., parallel to the axis of rotation of the polygon mirror.
Commonly assigned U.S. patent application Ser. No. 07/948,530, to Thomas L. Paoli, the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS system in which the rotating polygon mirror simultaneously deflects a plurality of orthogonally polarized and dissimilar wavelength laser beams having their largest divergence angles parallel to one another that are subsequently separated by a polarized beam separator and a plurality of dichroic beam separators and directed to their associated imaging stations. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar path lengths for each beam. This is facilitated by locating all lasers in one integral unit. The laser diodes are arranged in a line in a cross-scan direction (i.e., they are sagittally offset) and must be fabricated such that they are packed closely together in a direction parallel to the polygon mirror rotation axis to minimize beam characteristic deviations such as spot size, energy uniformity, bow and linearity. That is, the laser diodes are kept as close together as possible in the cross-scan direction so that the light beams strike as nearly the same portion of the polygon mirror as possible.
Commonly assigned, U.S. patent application Ser. No. 08/156,219 entitled "Offset Mounting of Nonmonolithic Multiwavelength Lasers" to Kovacs et al., the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS architecture in which the laser diodes produce laser beams of different wavelengths that are axially displaced from one another. The laser producing the beam having the shortest wavelength is closest to the F.theta. scan lens to create common focus of the beams on separate photoreceptors.
In U.S. patent application Ser. No. 08/156,219, a thermally conductive spacer is used to join two laser mounting surfaces, wherein the width of the spacer controls the separation of the laser mounting surfaces.
Commonly assigned U.S. patent application Ser. No. 08/156,222 to Kovacs et al., entitled "Laser Diode Arrays With Close Beam Offsets", discloses mounting a first and a second monolithic laser diode on a submount so that the laser stripes of the first and second laser diodes have close beam offsets. As shown in FIG. 7 of U.S. patent application Ser. No. 08/156,222, this allows alignment of two laser diodes on a submount with a close beam offset. However, U.S. patent application Ser. No. 08/156,222 does not disclose alignment in the same plane of more than two laser diodes all having a close beam offset.
With certain raster output scanner designs that use multibeam laser sources coupled with beam splitting techniques to address multiple xerographic stations, four aligned beams having a sagittal separation with a close offset between the beams are used. The beams may be of four different wavelengths. Alternatively, the beams may be of two wavelengths with one of the beams of each of the two wavelengths having a different polarization from the other beam of the same wavelength.
Laser diode arrays are generally of two different varieties: monolithic and nonmonolithic. Monolithic arrays of laser diodes are arrays of laser stripes (i.e. layers of materials that lase when an electric current runs through them) that are produced as a unitary structure in the manufacturing process. By contrast, nonmonolithic arrays are structures that are not constructed as unitary arrays. Instead, a nonmonolithic array usually comprises a separate submount and a plurality of separate laser diodes that are coupled to the submount in some fashion such as solder, epoxy, or the like.
A problem with the monolithic approach is that there is a reduction of yield associated with each additional laser source because a separate regrowth step is required to produce each additional laser source. Thus, a four laser source array would require three regrowth steps, resulting in a relatively low yield. Fabricating a four laser source array using four separate laser diodes is labor intensive. It also is difficult to achieve the desired minimal spacing between laser sources when using four separate laser diodes.
Accordingly, there is a need for a laser array having four aligned, accurately and closely spaced lasers.